Hi, I watch your eev blog videos but this is my first post on your forum. Very cool to see this teardown. I'm a recreational paragliding pilot and also design variometers used for gliding that indicate your sink/climb rate. I actually have one of my open source audio-only designs in production as the "Microvario" at flytepark.com. Just a few weeks ago, someone asked me if it could be adapted for skydiving use. I did a few training AFF jumps in 1994/1995 (never graduated the course), and remember the cypress AAD from the skydiving mags back then ! A lot of current flight instruments are like this - late 80's designs, I have no idea how they maintain stock of the required components.
I can speculate on what it does. You would have to set the LZ /airport altitude, as barometric pressure sensors only measure pressure altitude, i.e. w.r.t. air pressure at sea level. That can change by +/- a hundred metres due to weather changes, so you need to set the ahem "bounce" altitude before you get into the aircraft. It would then measure two things, absolute altitude w.r.t. to the reference altitude you just set and the rate of change of altitude (obviously you don'
t want it going off when you're floating down under canopy). The way to do that is to sample the altitude at regular intervals (say several milliseconds) over a rolling window of time, e.g. the last couple of seconds worth of data, and do a linear regression of the altitude samples. That gives you the slope of the best fit straight line to the noisy altitude samples. This is your fall rate.
My answer to the guy who asked me whether my project could be adapted for skydiving is in your teardown video, and your comments - this is a life saving device, and would need a huge investment in calibration, certification and maintenance procedures. That's an analog device that would need a calibrated pressure chamber. These days you get $7 digital sensors e.g. MS5611 that have their calibration coefficients in ROM and are probably far more accurate and less noisy than a board level ADC circuit. Even then I wouldn't want to be responsible for certifying the design for life saving applications.
Random thought 1 : I have seen references in pressure sensor datasheets to Goretex type material that allow pressure sensors to sense atmospheric pressure, and still maintain device waterproof rating.
Random thought 2 : I know a skydiver, who takes his kit as airline cabin baggage - when he takes it through security, there is a laminated card from the manufacturer that he shows to the xray folks to convince them that the pyrotechnic device is safe and has been disabled via user menu, for the flight.
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